Transmission fluid

ABSTRACT

A transmission fluid contains a base oil and calcium carbonate. The calcium carbonate is dispersed in a form of aggregations in the transmission fluid. A ratio of the aggregations having a particle diameter of 200 nm or more is 28 mass % or less in terms of calcium based on a total amount of the aggregations. A base value of the transmission fluid is in a range from 0.5 mgKOH/g to 3 mgKOH/g by a hydrochloric acid method.

TECHNICAL FIELD

The present invention relates to a transmission fluid.

BACKGROUND ART

An automatic transmission (AT) is a transmission having a mechanism inwhich a transmission torque ratio is automatically set according to avehicle speed, a magnitude of load and the like. The automatictransmission includes a torque converter, gear mechanism, hydraulicmechanism, wet clutch and the like. Moreover, a continuously variabletransmission (CVT) is also often used as the transmission. A method oftransmitting torque by friction between a metallic belt or chain and ametallic pulley is well known. Further, in recent years, an automobileprovided with a Dual Clutch Transmission (DCT) has begun to come on themarket. In the DCT that is a kind of AT, dedicated clutches arerespectively prepared in an odd-numbered stage and an even-numberedstage. Since gears in a next stage are engaged in advance at gearchange, gear change can be quickly done just by switching the clutches.

Various transmission fluids usable for the above various transmissionshave been proposed. Particularly, a transmission fluid having a highkinematic friction coefficient (0) is required in order to increase atransmission torque volume. For instance, Patent Literature 1 disclosesa lubricating oil composition that contains a lubricating base oil,polyol compound, alkali metal borate, ashless dispersant and alkalineearth metal sulfonate. This composition can further enhance thekinematic friction coefficient (μd) of the wet clutch (see paragraph[0073] [Advantages of the Invention]).

CITATION LIST Patent Literature(s)

Patent Literature 1: JP-A-2005-8695

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In all of the above AT, CVT and DCT, with longer use of the transmissionfluid, the kinematic friction coefficient (μd) of the transmission fluidis decreased and vibration (shudder) of an entire vehicle body caused bystick-slip vibration of a clutch sliding portion is more likely tooccur. Accordingly, a longer clutch lifetime, specifically, a longershudder lifetime is demanded in practical use.

However, even the above transmission fluid disclosed in PatentLiterature 1 is not always sufficient for maintaining the kinematicfriction coefficient (μd) after the longtime use of the transmissionfluid. In addition, the transmission fluid has a complicated structure,which entails a high production cost.

An object of the invention is to provide a transmission fluid capable ofmaintaining a high kinematic friction coefficient (μd) for a long timeand providing a long clutch lifetime with a simple structure.

Means for Solving the Problems

In order to solve the above problem, the following transmission fluid isprovided according to an aspect of the invention.

According to the aspect of the invention, a transmission fluid contains:a base oil; and calcium carbonate, in which the calcium carbonate isdispersed in a form of aggregations in the transmission fluid, a ratioof the aggregations having a particle diameter of 200 nm or more is 28mass % or less in terms of calcium based on a total amount of theaggregations, and a base value of the transmission fluid is in a rangefrom 0.5 mgKOH/g to 3 mgKOH/g by a hydrochloric acid method.

According to the above aspect of the invention, a transmission fluidcapable of maintaining a high kinematic friction coefficient (μd) for along time and providing a long clutch lifetime with a simple structurecan be provided.

DESCRIPTION OF EMBODIMENT(S)

In an exemplary embodiment of the invention, a transmission fluid isobtained by blending a base oil with calcium carbonate, in which thecalcium carbonate is dispersed in a form of aggregations in thetransmission fluid, a ratio of the aggregations having a particlediameter of 200 nm or more is 28 mass % or less in terms of calciumbased on a total amount of the aggregations, and a base value of thetransmission fluid is in a range from 0.5 mgKOH/g to 3 mgKOH/g by ahydrochloric acid method. The transmission fluid of the invention(hereinafter, also referred to as “the present transmission fluid”) willbe described in detail below. Herein, the transmission fluid obtained byblending a base oil and calcium carbonate means not only a transmissionfluid containing the base oil and calcium carbonate, but also acomposition containing a modified substance obtained by modifying atleast one of the base oil and calcium carbonate, and a compositioncontaining a reactant obtained by reaction of the base oil and/orcalcium carbonate.

The base oil usable in the present transmission fluid is notparticularly limited, but may be at least one of the mineral oil and thesynthetic oil. Specifically, the base oil may be one or a plurality ofthe mineral oil, one or a plurality of the synthetic oil, or acombination of the mineral oil and the synthetic oil.

The mineral oil and the synthetic oil are not specifically limited, butany mineral and any synthetic oil generally usable as a base oil for atransmission are suitable. Particularly, the base oil having a kinematicviscosity at 100 degrees C. in a range from 1 mm²/S to 50 mm²/s,particularly from 2 mm²/s to 15 mm²/s is preferably usable. When thekinematic viscosity at 100 degrees of the base oil is 1 mm²/s or more,an increase in abrasion at a sliding portion such as a gear bearing anda clutch of the transmission is restrained. When the kinematic viscosityat 100 degrees of the base oil is 50 mm²/s or less, deterioration of alow-temperature viscosity is expected to be restrained.

A pour point of the base oil, which is an index of a low-temperaturefluidity, is not particularly limited, but is preferably minus 10degrees C. or less, particularly preferably minus 15 degrees C. or less.

Further, although not particularly limited, the base oil preferably hasa saturated hydrocarbon component of 90 mass % or more, a sulfur contentof 0.03 mass % or less and a viscosity index of 100 or more. When thesaturated hydrocarbon component is 90 mass % or more, an amount ofdeteriorated products is reducible. When the sulfur content is 0.03 mass% or less, an amount of deteriorated products is reducible. When theviscosity index of the base oil is 100 or more, abrasion at a hightemperature is reducible.

Examples of the mineral oil include a naphthenic mineral oil, aparaffinic mineral oil and GTL WAX. Specific examples of the mineral oilinclude light neutral oil, intermediate neutral oil, heavy neutral oil,and bright stock, which are obtainable by solvent purification orhydrogenation purification.

On the other hand, examples of the synthetic oil include polybutene, ahydride thereof, polyalphaolefin (e.g., 1-octene oligomer, 1-deceneoligomer), alkylbenzene, polyolester, diacid ester,polyoxyalkyleneglycol, polyoxyalkyleneglycolester,polyoxyalkyleneglycolether, hindered ester and silicone oil.

In the above base oil, it is preferable to blend (mix) polyalphaolefin(PAO) in use in order to efficiently achieve the advantages of theinvention. Examples of PAO include alphaolefin homopolymers andalphaolefin copolymers. A ratio of PAO is preferably 30 mass % or morein the base oil, preferably 50 mass % or more. A kinematic viscosity at100 degrees C. of PAO is preferably in a range from 2 mm²/s to 200mm²/s.

The present transmission fluid contains calcium carbonate in a specificform (hereinafter, also referred to as “the present calcium carbonate”).The present calcium carbonate is dispersed in a form of aggregations inthe transmission fluid. Among the aggregations, a ratio of anaggregation having a particle diameter of 200 nm or more (hereinafter,also referred to as a “large particle”) is 28 mass % or less in terms ofcalcium based on a total amount of the aggregations, preferably 25 mass% or less. When the ratio of the large particle in the aggregations is28 mass % or less, a resultant transmission fluid can maintain a highkinematic friction coefficient (μd) for a long time.

An average particle diameter of the above aggregations is preferably ina range from 1 nm to 180 nm, more preferably from 10 nm to 150 nm inorder to efficiently achieve the advantages of the invention.

The particle diameter, a particle-diameter distribution and the averageparticle diameter of the above aggregations are measurable according toelectrophoretic light scattering. For instance, a particle-diametermeasurement system using ELSZ-1000S manufactured by Otsuka ElectronicsCo., Ltd. is suitably usable.

In order to obtain the transmission fluid containing the present calciumcarbonate and the base oil, in which a predetermined amount of largeparticles are dispersed in the transmission fluid, for instance, thebase oil or any transmission fluid may be blended with calcium carbonatehaving an aggregations with a predetermined particle size distributionor may be blended with an overbased organic acid calcium salt compoundincluding a predetermined aggregation (e.g., an overbased detergent). Inthe latter case, the present calcium carbonate is derived from theoverbased organic acid calcium salt compound. A manufacturing method ofcalcium carbonate (aggregations) is not particularly limited, but, forinstance, may include neutralizing an organic acid with basic calciumoxide and hydroxide and subsequently carbonating excessive basic calciumoxide and hydroxide.

A content of the present calcium carbonate is preferably in a range from0.001 mass % to 0.3 mass % in terms of calcium based on a total amountof the present transmission fluid, more preferably from 0.001 mass % to0.2 mass %, further preferably from 0.01 mass % to 0.2 mass %. When thecontent of the present calcium carbonate falls within the above range, ahigh kinematic friction coefficient (μd) can be maintained for a longtime.

The ratio of the large particles in the present transmission fluid ispreferably 0.056 mass % or less in terms of calcium based on the totalamount of the present transmission fluid, more preferably from 0.045mass % or less. When the ratio of the large particle in the presenttransmission fluid falls within the above range, a high kinematicfriction coefficient (μd) can be maintained for a long time.

Among the above-described overbased organic acid calcium salt compound,the overbased detergent is preferably at least one of a sulfonatedetergent, a salicylate detergent and a phenate detergent in order toefficiently achieve the advantages of the invention.

A base value of the present transmission oil is in a range from 0.5mgKOH/g to 3 mgKOH/g by a hydrochloric acid method (JIS K2501), morepreferably in a range from 0.7 mgKOH/g to 2.8 mgKOH/g. When the basevalue falls within the above range, a high kinematic frictioncoefficient (μd) can be maintained for a long time.

The overbased organic acid calcium salt compound (overbased detergent)itself only needs to contain calcium carbonate and be overbased. Thebase value of the overbased organic acid calcium salt compound is notparticularly limited, but a total base value thereof is preferably in arange from 10 mgKOH/g to 400 mgKOH/g by the hydrochloric acid method(JIS K2501).

The present transmission fluid preferably has the kinematic viscosity at100 degrees C. in a range from 3 mm²/s to 8 mm²/s, more preferably in arange from 4 mm²/s to 7 mm²/s in order to efficiently achieve theadvantages of the invention. A viscosity index of the presenttransmission fluid is preferably 100 or more.

The present transmission fluid can contain various additives as long asan object of the invention is not hampered. Examples of the additivesusable as needed include a viscosity index improver, an antioxidant, aantiwear agent, a friction modifier, an ashless dispersant, a metaldeactivator, a rust inhibitor an antifoaming agent, a pour pointdepressant, a surfactant and a coloring agent.

Examples of the viscosity index improver include polymethacrylate,dispersed polymethacrylate, olefin copolymer (e.g. ethylene-propylenecopolymer), dispersed olefin copolymer and styrene copolymer (e.g.styrene-diene copolymer and styrene-isoprene copolymer). A content ofthe viscosity index improver is approximately in a range from 0.5 mass %to 15 mass % of the total amount of the present transmission fluid inview of the blending effect thereof.

An example of the pour point depressant is polymethacrylate having amass average molecular weight of 10000 to 150000. A preferable contentof the pour point depressant is approximately in a range from 0.01 mass% to 10 mass % of the total amount of the present transmission fluid.

Examples of the antioxidant include an aminic antioxidant, a phenolicantioxidant and a sulfuric antioxidant.

Examples of the aminic antioxidant may include monoalkyldiphenylaminecompounds such as monooctyldiphenylamine and monononyldiphenylamine;dialkyldiphenylamine compounds such as 4,4′-dibutyldiphenylamine,4,4′-dipentyldiphenylamine, 4,4′-dihexyldiphenylamine,4,4′-diheptyldiphenylamine, 4,4′-dioctyldiphenylamine and4,4′-dinonyldiphenylamine; polyalkyldiphenylamine compounds such astetrabutyldiphenylamine, tetrahexyldiphenylamine,tetraoctyldiphenylamine and tetranonyldiphenylamine; and naphthylaminecompounds such as alpha-naphthylamine, phenyl-alpha-naphthylamine,butylphenyl-alpha-naphthylamine, pentylphenyl-alpha-naphthylamine,hexylphenyl-alpha-naphthylamine, heptylphenyl-alpha-naphthylamine,octylphenyl-alpha-naphthylamine and nonylphenyl-alpha-naphthylamine.Particularly, the compounds having the alkyl group having 4 to 24 carbonatoms, particularly preferably 6 to 18 carbon atoms are usable. One ofthe aminic antioxidant as described above may be used alone or two ormore thereof may be used in combination.

Examples of the phenolic antioxidant may include 2,6-di-t-butylphenol,2,6-di-t-butyl-4-methylphenol, 4,4′-methylenebis(2,6-di-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol),2,2′-methylenebis(4-ethyl-6-t-butylphenol),2,2′-methylenebis(4-methyl-6-t-butylphenol), 4,4′-isopropylidenebisphenol, 2,4-dimethyl-6-t-butylphenol,tetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane,1,1,3-tris(2-methyl-4-hydroxy5-t-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, and2,6-di-t-butyl-4-ethylphenol.

Examples of the sulfuric antioxidant may includedialkylthiodipropionate, dialkylthiocarbamic acid derivative (except fora metal salt), bis(3,5-di-t-butyl-4-hydroxybenzyl)sulfide,mercaptobenzothiazole, a reactant of phosphorus pentasulfide and anolefin, and dicetyl sulfide.

One of various antioxidants as described above may be used alone or twoor more thereof may be used in combination. Particularly, the aminicantioxidant, the phenolic antioxidant or zinc alkyldithio phosphate ispreferably used. A preferable content of the antioxidant isapproximately in a range from 0.05 mass % to 3 mass % of the totalamount of the present transmission fluid.

Examples of the antiwear agent may include a thiophosphoric acid metalsalt (e.g., Zn, Pb and Sb), a thiocarbamic acid metal salt (e.g., Zn), asulfur compound, phosphate ester (tricresyl phosphate) and phosphiteester. A preferable content of the antiwear agent is approximately in arange from 0.05 mass % to 5 mass % of the total amount of the presenttransmission fluid.

Examples of the friction modifier may include a polyhydric alcoholpartial ester such as neopentyl glycol monolaurate, trimethyrolpropanemonolaurate, glycerin monooleate (oleic acid monoglyceride). Apreferable content of the friction modifier is approximately in a rangefrom 0.05 mass % to 4 mass % of the total amount of the presenttransmission fluid.

Examples of the ashless dispersant may include succinimides,boron-containing succinimides, benzylamines, boron-containingbenzylamines, succinic acid esters, and mono- or di-carboxylic acidamides respectively represented by a fatty acid or succinic acid. Apreferable content of the ashless dispersant is approximately in a rangefrom 0.1 mass % to 20 mass % of the total amount of the presenttransmission fluid.

One of the metal deactivators such as benzotriazole and thiadiazole maybe used alone or two or more thereof may be used in a combination. Apreferable content of the metal deactivator is approximately in a rangefrom 0.01 mass % to 5 mass % of the total amount of the presenttransmission fluid.

Examples of the rust inhibitor may include a fatty acid, alkenylsuccinicacid half ester, fatty acid soap, alkyl sulfonate, fatty acid ester ofpolyhydric alcohol, fatty acid amide, oxidized paraffin and alkylpolyoxyethylene ether. A preferable content of the rust inhibitor isapproximately in a range from 0.01 mass % to 3 mass % of the totalamount of the present transmission fluid.

One of the antifoaming agents such as a silicone compound and an estercompound may be used alone or two or more thereof may be used in acombination. A preferable content of the antifoaming agent isapproximately in a range from 0.05 mass % to 5 mass % of the totalamount of the present transmission fluid.

The pour point depressant is exemplified by polymethacrylate. Apreferable content of the pour point depressant is approximately in arange from 0.01 mass % to 10 mass % of the total amount of the presenttransmission fluid.

The surfactant is exemplified by polyoxyethylene alkyl phenyl ether. Apreferable content of the surfactant is approximately in a range from0.01 mass % to 10 mass % of the total amount of the present transmissionfluid.

The present transmission fluid as described above provides a long clutchlifetime since the present transmission fluid can maintain a highkinematic friction coefficient (μd) for a long time, and is suitablyapplicable to various transmissions such as an automatic transmission(AT), a continuously variable transmission (CV) and a dual clutch (DCT).

EXAMPLES

Next, the invention will be described in more detail with reference toExamples and Comparatives. It should be noted that the invention is notlimited to description of the examples and the like.

Example 1 and Comparatives 1 to 2

PAO (a kinematic viscosity at 100 degrees C. of 4.0 mm²/s) was used asthe base oil. An overbased calcium sulfonate having properties shown inTable 1 was blended at 0.15 mass % in terms of calcium based on a totalamount of each of the sample oils to prepare sample oils respectivelyhaving properties shown in Table 1 (all the sample oils are intended tobe used for the transmission). The viscosity index of each of the sampleoils in Example 1 and Comparatives 1 and 2 was 120.

The overbased calcium was dispersed in the base oil and checked in termsof a structure of the aggregations. Specifically, a dispersed state ofthe aggregations (including large particles) in each of the sample oilswas measured by ELSZ-1000S manufactured by Otsuka Electronics Co., Ltd.A ratio (mass % relative to the aggregations) of the large particles inthe aggregations formed of calcium carbonate and an average particlediameter of each of the large particles were calculated. The results areshown in Table 1. Average particle diameters of the sample oils wererespectively 100 nm in Example 1, 200 nm in Comparative 1 and 800 nm inComparative 2.

TABLE 1 Sample Oil Kinematic Base value by Calcium Carbonate viscosityat hydrochloric Rate of large Sample Oil 100° C. acid method particlesTest time (hr) and Kinematic friction coefficient (μd) (mm²/s) (mgKOH/g)(mass %) 0 1 6 12 24 48 72 96 120 Ex. 1 4 1.39 23 0.123 0.127 0.1270.127 0.130 0.131 0.132 0.135 0.134 Comp. 1 4 3.90 40 0.122 0.121 0.1250.130 0.118 0.138 0.111 0.085 0.048 Comp. 2 4 1.07 73 0.102 0.129 0.1430.149 0.145 0.150 0.143 0.049 0.009

Evaluation Method

A kinematic friction coefficient of each of the sample oils was measuredunder the following test conditions using an LVFA friction tester inaccordance with JASO M349-2001. The results are shown in Table 1.

Material: a cellulose clutch material used in an actual transmission

Face Pressure: 1.0 MPa

Oil Temperature: 120 degrees C.

Rotation Speed: 150 rpm

Evaluation Results

As is understood from the results shown in Table 1, the sample oil ofExample 1 containing substantially no large particle (aggregationshaving a particle diameter of 200 nm or more) can maintain a highkinematic friction coefficient (μd) for a long time and provide a longclutch lifetime (shudder lifetime). On the other hand, in the sampleoils of Comparatives 1 and 2 having a lot of large particles, althoughthe sample oils having an approximately equivalent content of overbasedcalcium sulfonate and an approximately equivalent base value, thekinematic friction coefficient (μd) is significantly decreasedapproximately after the elapse of 72 hours after the test starts.Accordingly, it is understood that the decrease in the large particlesin the transmission fluid is crucial.

1. A transmission fluid comprising: a base oil; and calcium carbonate,wherein the calcium carbonate is dispersed in a form of aggregations inthe transmission fluid, a ratio of the aggregations having a particlediameter of 200 nm or more is 28 mass % or less in terms of calciumbased on a total amount of the aggregations, and a base value of thetransmission fluid is in a range from 0.5 mgKOH/g to 3 mgKOH/g by ahydrochloric acid method.
 2. The transmission fluid according to claim1, wherein the calcium carbonate is derived from an overbased organicacid calcium salt compound, and a content of the calcium carbonate is arange from 0.001 mass % to 0.3 mass % in terms of calcium based on atotal amount of the transmission fluid.
 3. The transmission fluidaccording to claim 2, wherein the overbased organic acid calcium saltcompound is an overbased detergent, and the overbased detergent is atleast one selected from the group consisting of a sulfonate detergent, asalicylate detergent and a phenate detergent.
 4. The transmission fluidaccording to claim 1, wherein the base oil comprises polyalphaolefin. 5.The transmission fluid according to claim 4, wherein a ratio of thepolyalphaolefin in the base oil is 30 mass % or more.
 6. Thetransmission fluid according to claim 4, wherein the polyalphaolefin hasa kinematic viscosity at 100 degrees C. in a range from 2 mm²/s to 200mm²/s.
 7. The transmission fluid according to claim 1, wherein thetransmission fluid has a kinematic viscosity at 100 degrees C. in arange from 3 mm²/s to 8 mm²/s, and the transmission fluid has aviscosity index of 100 or more.
 8. The transmission fluid according toclaim 1, wherein the transmission fluid is suitable for an automatictransmission or a continuously variable transmission.